An SCM-only user has no groups, so do not throw an undocumented exception looking them up.
… hudson.ExpressionFactory2$JexlExpression evaluate
WARNING: Caught exception evaluating: it.authorities in /user/demo/. Reason: java.lang.reflect.InvocationTargetException
java.lang.reflect.InvocationTargetException
	at …
Caused by: org.acegisecurity.userdetails.UsernameNotFoundException: demo
	at hudson.security.SecurityRealm$None$2.loadUserByUsername(SecurityRealm.java:494)
	at jenkins.security.ImpersonatingUserDetailsService.loadUserByUsername(ImpersonatingUserDetailsService.java:32)
	at hudson.model.User.impersonate(User.java:282)
	at hudson.model.User.getAuthorities(User.java:769)
	... 93 more